1. Field of the Invention
The present invention relates to a MEMS and, more particularly, to a MEMS microphone.
2. Description of the Related Art
A micro-electromechanical system (MEMS) is a microscopic machine that is fabricated using the same types of steps (e.g., the deposition of layers of material and the selective removal of the layers of material) that are used to fabricate conventional analog and digital CMOS circuits.
For example, one type of MEMS is a microphone. Microphones commonly use a micro-machined diaphragm (a thin layer of material suspended across an opening) that vibrates in response to pressure changes (e.g., sound waves). Microphones convert the pressure changes into electrical signals by measuring changes in the deformation of the diaphragm. The deformation of the diaphragm, in turn, can be detected by changes in the capacitance, piezoresistance, or piezoelectric effect of the diaphragm.
FIG. 1 shows a view that illustrates a prior-art, piezoelectric microphone 100. As shown in FIG. 1, microphone 100 includes a rigid U-shaped back plate 110, a diaphragm 112 that is formed across the opening in back plate 110, and a piezocrystal 114 that is connected between back plate 110 and diaphragm 112.
In operation, changes in air pressure (e.g., sound waves) cause diaphragm 112 to vibrate which, in turn, causes the end of piezocrystal 114 to be pushed and pulled. The pushing and pulling on the end of piezocrystal 114 oppositely charges the two sides of piezocrystal 114. The charges are proportional to the amount of pushing and pulling, and thus can be used to convert pressure waves into electrical signals which can then be amplified.
When a microphone is reduced in size to that of a MEMS, one concern that arises is sensitivity. This is because the size of the diaphragm of a MEMS microphone is so relatively small (e.g., less than a millimeter across), due to being formed across a cavity or a back side opening in a relatively-small semiconductor die.